my ($pat, $repl, $mods) = $substitution =~ m{
    ^s/
    ( (?:[^/\\]+|\\.)* )
    /
    ( .* )
    /
    ( [a-z]* )
    \z
}xs
    or die("Bad substitution\n");

my %mods = map { $_ => 1 } $mods =~ /./sg;
my $global = delete($mods{g});
$mods = join('', delete(@mods{qw( i m s x )}))
   and $pat = "(?$mods:$pat)";

die("Unknown modifiers \"", keys(%mods), "\"\n")
    if %mods;

no re 'eval';
if ($global) {
    $candidate =~ s/$pat/$repl/g;
} else {
    $candidate =~ s/$pat/$repl/;
}